Heat dissipation in a microelectronic device, such as a microprocessor results in high operating temperature of its semiconductor (e.g. Silicon) substrate. It requires creative means of cooling in order to contain substrate's temperature. The thermal flux density (Watts/m2) has both spatial and temporal (time) dependence. Present microprocessors dissipate about 30-40 Watts/cm2. Thermal flux density at a location has a steady component due to leakage current and a dynamic component (often with transients or spikes) due to application driven digital switching. Typical thermal flux density is non-uniformly distributed over a chip. The thermal flux density over a “hotspot” can be 5 times higher than the remaining area on a chip. As cooling of a microprocessor becomes increasingly difficult under high power environment, the need to manage complex temperature distribution of a chip becomes even more critical.
Reliability of microelectronic devices, such as microprocessors, is known to be affected by the operating temperature of its substrate. Under steady temperature condition it has been shown that the life time of a microelectonic device is exponentially reduced with an increase in its temperature. However, the effect of temperature variations on reliability is not well established. Nevertheless, controlling the temperature variation as well as steady temperature of a microprocessor chip below a target temperature is much preferable to a case with more extreme fluctuations.
A silicon substrate of a microelectronic device may, for example, be 500-700 μm thick with an area spanning about 2 cm×2 cm. The circuit elements are often fabricated on one side of the surface and within a total thickness of about 100 μm from the substrate surface. The substrate has a “top surface” on to which a cooling device is attached and a “bottom surface” on which the microelectronic circuits are built. A microprocessor includes a number of the microelectronic circuits including several logical units, such as floating point, load-store-registers, etc., that perform the necessary digital operations. Among these logical units, a subset is known to produce more heat flux than others. During a transient periods of operation of these logical units, the local temperature at these units can exceed the temperature of the neighboring elements for short periods of time. More than 80% of the heat generated by circuit elements, such as these logical units, travel through the bulk thickness of the silicon substrate towards a cooling device such as a heat spreader or a heat sink. Present commercial products envisage minimizing the resistance to heat transfer by thinning the silicon. However, to maintain robustness of a chip it is important to maintain a minimum thickness to avoid damage due to internal and external mechanical stresses.
The invention provides a solution to improving the thermal characteristics of a chip by modifying the thermal properties of a substrate (for example, silicon) without interfering with the chip process technology or reducing the robustness of the chip.
A via hole is a cavity (often cylindrical) which extends, partly into or completely through, an insulating or semiconducting substrate. A blind via hole is a via hole that extends partly into the insulating or semiconducting substrate. For the purpose of the present invention, a via is a conducting body formed by a via hole which contains at least a partial filling of electrically or thermally conducting material.